Semiconductors are commonly formed in a plasma processing chamber where etching and deposition are enhanced by plasma generated ions that bombard the wafer substrate. The plasma can be ignited and sustained via radio frequency (RF) power from an RF source inductively or capacitively coupled into the plasma via application of the RF between a first electrode and a ground (e.g., the chamber walls). A wafer or substrate to be processed can rest atop or be coupled to a second electrode (optionally biased or grounded) for processes that include wafer cleaning, ion implantation, etching, coating or substrate modification, and ion-enhanced deposition, to name a few. For reactive ion etching (RIE) the substrate can be coupled to the first electrode. A frequency and magnitude of the RF source can influence plasma density, which is one ‘knob’ in many semiconductor processing recipes since plasma density influences a number and rate of ions that impact the substrate, and hence influence cleaning, etching, etc.
A sheath voltage is another ‘knob’ that can be used to control ion energy in many recipes. A sheath or dark space exists between the plasma and any solid surface, such as the substrate or the chamber walls. The sheath is largely devoid of electrons as these get accelerated into the plasma, and thus a voltage difference, between the plasma and any solid surface, across the sheath exists, which accelerates ions that enter the sheath and causes the ions to bombard any solid surface on an opposite side of the sheath from the plasma. Thus, the sheath potential, or potential difference between the plasma and the substrate, controls an ion bombardment energy or ion energy. Ion energy influences how much energy each incoming ion transfers to the substrate upon impact, which can influence a rate of etching as well as an etching profile (a profile of the feature etched into the substrate).
While the second electrode can be grounded, in some instances, a DC bias is applied to the second electrode in order to control the ion energy. However, because of cross coupling of the RF to the second electrode, it is typically difficult to control ion energy independent of the plasma density. In other words, the RF source produces some DC bias on the substrate via RF cross coupling and this DC bias affects a minimum ion energy even where no intentional DC bias is applied to the second electrode.
Yet, some processes call for an ion energy that is lower than this minimum. One solution is to include an LC resonant circuit between the second electrode and the DC bias that is tuned to dampen and reduce the RF cross coupling. For instance, including a variable capacitor between the second electrode and the DC bias can create an inherent series LC resonant circuit due to inherent inductances in the wires and circuitry. Theoretically, tuning the variable capacitor such that a minimum of the LC resonant circuit is achieved can minimize the cross coupled RF field at the substrate, which results in a lower DC bias, and in turn enables lower minimum ion energies.
However, tuning of such LC resonant circuits has not produced the expected reductions in the cross coupled RF field, such systems are highly unstable, and real-time tuning is not possible.